1. Field of the Invention
The present invention relates to a scanning lens and an optical scanner that uses it. More particularly, the invention relates to a single-element scanning lens for use as an f.theta. lens in laser scanning optics, as well as an optical scanner equipped with this scanning lens.
2. Description of the Related Art
Optical scanners are extensively used with optical printers, digital copiers and the like. Such optical scanners employ a deflector such as a polygonal mirror for deflecting incident light at a constant angular velocity in a direction corresponding to the main scanning direction, and an f.theta. lens as a scanning lens. The f.theta. lens has two functions; according to one function, a laser beam that issues typically from a semiconductor laser device and which has been deflected by the deflector is focused as a spot on a scanning surface such as a photoreceptor drum or belt and the other function is to move the beam spot over the scanning surface at a constant speed.
In most cases, the f.theta. lens is provided with the capability by which the position of a deflecting point on the deflector and the position of a beam spot on the scanning surface are rendered to satisfy a conjugated relationship in a plane crossing at right angles with the plane formed by the deflected laser beam. A cylindrical lens is positioned on the side of the deflector where the laser beam is launched and it has a lens power in a direction corresponding to the sub-scanning direction. The f.theta. lens combines with this cylindrical lens to constitute tilt-correcting optics which corrects optically any inclination of reflecting surfaces of the deflector and which is capable of producing a substantially circular beam spot.
Many of the commercial f.theta. lenses available today are optics that are composed of two or more lens elements but they suffer from the disadvantage of complexity in construction. To solve this problem, f.theta. lenses composed of only one lens element have been proposed as simple optics.
Japanese Patent Unexamined Publication Nos. Sho. 57-144518, Sho. 63-50812 and Hei. 3-55513 teach one-element f.theta. lenses having a toric surface on one side and a spherical, cylindrical or toric surface on the other side. Japanese Patent Unexamined Publication Nos. Sho. 62-138823, Hei. 4-50908 and Hei. 5-45580 teach f.theta. lenses having an aspheric surface.
However, the conventional f.theta. lenses have their own problems. The f.theta. lens described in Japanese Patent Unexamined Publication No. Sho. 57-144518 which is solely composed of toric surfaces provides no better performance than a single spherical lens element when compared on the main scanning surface and, hence, it suffers from the disadvantage of small latitude in design, limited extent to which imaging performance can be insured in terms of f.theta. characteristics and curvature of field, and the resulting failure to achieve high image resolution. If one strains to assure satisfactory imaging performance, the center thickness of the lens has to be increased as described in Japanese Patent Unexamined Publication Nos. Sho. 63-50812 and Hei. 3-55513 but this only results in increased difficulty in lens manufacture.
With a view to eliminating these drawbacks, Japanese Patent Unexamined Publication Nos. Sho. 62-138823 and Hei. 4-50908 have proposed f.theta. lenses that adopt an aspheric surface in order to provide improved imaging performance. However, these f.theta. lenses are so complex in shape as to have no rotating axis and, hence, they are very difficult to design and manufacture. The f.theta. lens described in Japanese Patent Unexamined Publication No. Hei. 5-45580 is such that a cross-sectional shape taken in the main scanning direction is of a meniscus convex to the scanning surface and it adopts an aspheric surface having a rotating axis. However, light condensing optics must be provided on the side of a tenon-shaped mirror (deflector) where incident light is launched. In addition, the f.theta. characteristics of the lens are substantially associated with constant speed and electrical correction must be made to achieve linearity but, then, the overall equipment becomes complicated and costly.